In recent years, there has been an increasing interest in distributed antenna arrays due to their potential to provide improvements in performance, scalability, robustness, and cost over classic phased antenna arrays. Distributed arrays synthesize a large aperture using small, low-cost, and low-power devices, supporting improvements that would otherwise be too costly or bulky to achieve in a single system. Such arrays also have the flexibility to be scaled by adding or removing elements from the array, depending on the application at hand. Within distributed antenna arrays, there are generally two classes that are considered: incoherent distributed arrays, where little or no coordination is performed between nodes in the array, yielding a collection of individual wireless systems; and coherent distributed arrays, where element coordination is performed at the level of the radio frequency phase. While incoherent arrays are easier to implement, their improvements, such as gain and signal-to-noise ratio, generally scale only as the square-root of the number of elements, yielding diminishing returns. Coherent arrays achieve sensitivity improvements directly proportional to the number of elements in the array, yielding significant improvements as the array scales. However, distributed coherence requires significantly more coordination between nodes. The electrical states that need to be aligned to enable coherent beamforming include: each device's internal clock frequencies; relative timing of information symbols; and alignment of the beamforming phase. In general, there are two methods to achieve alignment: closed-loop and open-loop. Closed loop is only feasible to applications that have reliable feedback from the receive location, such as communications systems. Open-loop requires the nodes to coordinate without feedback, but opens the application space to instances where there is no feedback from the destination such as radar and remote sensing.In this work, I focus on the alignment of the phase of the beamforming signals in open-loop coherent distributed antenna arrays. I present a distributed antenna array supporting open-loop distributed beamforming at 1.5 GHz. Based on a scalable, high-accuracy internode ranging technique, I demonstrate open-loop beamforming experiments using three transmitting nodes. To support distributed beamforming without feedback from the destination, the relative positions of the nodes in the distributed array must be known with accuracies below $\lambda/15$ of the beamforming carrier frequency to ensure that the array maintains at least 90% coherent beamforming gain at the receive location. For operations at microwave frequencies, this leads to range estimation accuracies of centimeters or less. I present scalable, high-accuracy waveforms and new approaches to refine range measurements to significantly improve the estimation accuracy. Using one of the designed waveforms with a three-node array, I demonstrate high-accuracy ranging simultaneously between multiple nodes, from which phase corrections on two secondary nodes are implemented to maintain beamforming with the primary node, thereby supporting open-loop distributed beamforming. Upon movement of the nodes, the range estimation is used to dynamically update the phase correction, maintaining beamforming as the nodes move. I show the first open-loop distributed beamforming at 1.5 GHz with two-node and three-node arrays, demonstrating the ability to implement and maintain phase-based beamforming without feedback from the destination.
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